skull in many cetaceans. The Crustacea, however, present numerous 

 instances of asymmetrical development; for example, in the general 

 form of Bopjjrns and Pel'ogaster, and in th(j claws of many decapods. 



We are not aware that any cases of asymmetry have been re- 

 corded among the worms; and certainly very few among insects; 

 there are occasionally slight differences in the right and left mandibles 

 of some mandibulates, and Coccus has recently been referred to by 

 Gerstaecker^ as an example of asymmetry, without further specifica- 

 tion; we have been unable to discover to what he refers. Loew, also, 

 in the first of his monographs of the North American Diptera, states 

 that the hypopygium (the extci-nal genital armature) of the males of 

 SyrphidsE and Pipunculidaj is unsymmetrical. We append a figure 

 of this organ in an American species of Phora, closely allied to, if not 

 identical with, P. mlcroccpliala Loew, in which the left clasp (c) is 

 very much stouter and somewhat longer than the right one (c') ; the 

 drawing is magnified thirty five times. 



In a i-ecent study of the external genital organs of the males of 

 butterflies, we chanced to examine those of certain native species of 

 Nisoniades, and found not only a great difference between allied 

 species, but a most remarkable asymmetry between opposite clasps of 

 the same individual ; this has led to an examination of all the North 

 American species of which avc could obtain specimens for dissection, 

 and the results are embodied in this paper. The species have been 

 ibund to be much more numerous than was anticipated ; and yet we 

 are aware of several others from distant parts of North America, 

 whicli T\'e have not been able. to obtain; we trust that the hew inter- 

 est, which will hereafter attach to this otherwise inconspicuous group, 

 will lead to important results."-^ 



By referring to Fig. 15 of N. trhtis Boisd., which we will use for 

 illustration,^ it will be seen that the clasps (15. R., 15. L.) in this 

 genus are developed to an unusual degree, and exhibit more clearly 

 than the central organ the prevailing asymmetry of the parts. With 

 some minor exceptions, which will be specified below, the left clasp 

 is always more highly developed than the right, both in the con- 

 figuration of the whole, and in the sculpture and armature of the 

 details; each clasp may be divided, for convenience of description, 



1 Broun, Klassen u. Ordu. d. Thierreichs, v, 33. 



' Tbe same organs are asymmetrical in the closely allied genus Acliylodes. 



* This, and all the succeeding drawings, are magnified twenty diameters. 



